1. Technical Field
The present invention relates to a waste water treating method, appratus and system for waste water which contains phosphoric acid, a phosphorus compound, phosphate ions, organic nitrogen, nitrite nitrogen, nitrate nitrogen and ammonia nitrogen.
2. Background Art
It has conventionally been well known that existence of a nitrogen compound and a phosphorus compound is one of causes of eutrophication of rivers and lakes. It is also well known that the phosphorus compound and the nitrogen compound exist in domestic waste water and industrial waste water in large amounts and are difficult to purify. Heretofore, as a purification treatment for the nitrogen compound, a biological treatment has been generally implemented. The biological treatment comprises two steps, i.e. a nitrification step for converting ammonia nitrogen to nitrate nitrogen and a denitrification step for converting nitrate nitrogen to nitrogen gas.
Meanwhile, a variety of techniques for treating the phosphorus compound have been proposed, and a lime agglomeration/precipitation technique is known for domestic waste water. This is a technique for removing phosphate ions in water to be treated (hereinafter, “water to be treated” will be referred to as “for-treatment water”) by reacting the phosphate ions with calcium ions so as to condense and precipitate the phosphate ions as calcium hydroxyapatite which is a water-insoluble salt.
However, the conventional biological treatment has problems that it requires two different reaction vessels and that because the treatment proceeds slowly, its treatment efficiency is low. Further, the conventional technique has a problem that it requires a large-capacity apparatus since it does not treat a nitrogen compound and a phosphorus compound which are contained in for-treatment water simultaneously.
Further, in the biological treatment, there is another problem that large-capacity aerobic and anaerobic vessels are required for keeping nitrifying bacteria and denitrifying bacteria, thereby inducing an increase in equipment construction costs and an apparatus installation area. There is still another problem that since the denitrifying bacteria are largely influenced by an ambient temperature environment, components contained in for-treatment water and the like, and in particular, during the winter season when the temperature is low, their activities are lowered to deteriorate the denitrifying action, resulting in unstable processing efficiency.
In addition, in the case of the technique for treating the phosphorus compound, since a pH of the treated water after the condensation/precipitation of the phosphorus compound is high, the alkaline treated water resulting from the treatment of the phosphorus compound must be neutralized. Further, the technique also has a problem that maintenance is difficult due to a large amount of lime used.
Under the circumstances, the present invention has been invented to solve the technical problems of the prior art and proposes a waste water treating method, a waste water treating appratus and a waste water treating system which are capable of treating a nitrogen compound and a phosphorus compound in the same treating vessel and treating waste water containing a nitrogen compound and a phosphorus compound efficiently.